Hydraulic-power control device for power-assisted steering system

ABSTRACT

Herein disclosed is a hydraulic-power control device of a servo mechanism for automotive power-assisted steering systems. The control device includes a booster unit by which the working fluid pressure to be directed to the reaction chamber of the servo mechanism is multiplied depending upon several ranges of the vehicle driving speed so as to provide effortless steering performance during relatively low-speed driving and stabilized steering performance during relatively high-speed driving especially under the straight-ahead condition of the steering system.

United States Patent 1191 1111 3,777,839

Uchiyama et al. Dec. 11, 1973 HYDRAULIC-POWER CONTROL DEVICE I [56] References Cited FOR POWER-ASSISTED STEERING UNITED STATES PATENTS SYSTEM 2,893,504 1/1959 Jackson ISO/79.2 R [75] Inventors: Hiromichi Uchiyama, Tokyo; fi 0 ey a??? lnque Yok0hama both of 3,085,645 4/1963 Bookout et al. 120/792 R P 3,465,842 9/1969 l-lruska 180/792 R [73] Assignee: Nissan Motor Company, Limited,

Yokohama City, Japan Primary Examinerl(enneth H. Betts Assistant ExaminerLeslie J. Paperner [22] Filed. Dec. 22, 1971 Attorney john Lezdey 211 Appl. No.: 210,809

[57] ABSTRACT [30] Foreign Application p i i Data Herein disclosed is a hydraulic-power control device of a servo mechanism for automotive power-assisted 3:2 33 2 2: steering systems. The control device includes a 1971 H4288 booster unit by which the working fluid pressure to be 197] H4504 directed to the reaction chamber of the servo mecha- 1971 J 46/1458) nism is multiplied depending upon several ranges of ap the vehicle driving speed so as to provide effortless [52] U S 180/79 2 R 91/434 steering performance during relatively low-speed driv- [Sl] B62d 5/08 ing and stabilized steering performance during rela- [58] Fie'ld T 60525 tively high-speed driving especially under the straight- 371, 3172, 3373 ahead condition of the steering system.

29 Claims, 13 Drawing Figures P u M P 4 I 4 PS I BOOSTER STEER I N G l PRESSURE CONT. VALVE pd CONT. VALVE I I I 2 VEHICLE SPEED CONTROL DETECTOR CIRQU 1 REACTION B CHAMBER PATENTEDDEC I 1 I973 3.777.839 SEE! 010? 11 F/g. PRIOR ART A PUMP F. C STEERING STEERING PO PRESSURE I RESIST NCE CONT vAI vE CONT. VALVE PC D E VEHICLE SPEED SPEED 7 DETECTOR MODULE I /B STEERING REACTION REACTION CHAMBER PP PUMP T A4 PS I3 BOOSTER 1 STEERING 1 PRESSURE CONT. VALVE H pd CONT. VALVE l2 VEHICLE SPEED CONTROI DETECTOR CIRCUIT REACTION B CHAMBER INVENTORS HIRO-MICHI UCHIYAM 'V'NAOIHKO [NONE ATTORNEY .mgmgnuu; T 1 I915 3.777.839

SREU 020? 11 RESERVOIR PUMP I4 Z A H {D I8 343 F6 21 22c 22 IO\ 19 I STEERING STEERING P0 PRESSURE PS RESISTANCE CONT VALVE CONT. VALVE 1| l2 l l3 VEHICLE SPEED CONTROL SPEED DETECTOR CIRCUIT STEERlNG REACTION REACTlON CHAMBER 1N VENTOR S HIKOMICH/ UCHIYAMA f NADHIK INUUE BY hfl ATTORNEY PATENTEUUEC 1 191s SHEET 0311i 11 n. wm mmmmi E 3wa Q25 STEERING-ASSIST. PRESSURE, PG

Fig. 5

lEOnEm ozimmkm STEERI NG RESISTANCE INVENTORS HIROMICHI uCHIYfi /h'! NAUHIKO I ATTORNEY PATENIEDBEC n ma 3.777.839

BOOSTER 4 UNIT 50 58 43 H Y V 38 IN VE NTORS H ROMICHI U H/ YAMA' f NflOH/HO [NONE ATTORNEY wimmnzun 191s 3.777.839

sum 05 or 11 INVENTORS H/ROMICHI ucmv MA-f NAOH/k llv u' ATTORNEY PAIENIEnnzc 1 1 ms 3377.839

SIEU 030? 11 --M/ \NUAL STEERING Z O g I U 55 c a: d b

0 (CONVENTIONALLY ACHiEVED) JEERlNG RESISTANCE INVENTORS H/ROMICHI UCHIYAMA 'NAOHIK IN HE BY W? ATTORNEY HYDRAULIC-POWER CONTROL DEVICE FOR POWER-ASSISTED STEERING SYSTEM The present invention relates generally'to steering systems of motor vehicles and has a particular reference to a hydraulic-power control device for use in automotive power-assisted steering systems.

A majority of the modern motor vehicles are equipped with power-assisted steering systems for the purpose of lessening the physical effort required on the driver in operating the steering, so as to relieve the driver from fatigue and to enable him to softly and efficiently steer the vehicle especially when parking at a curb or when the traffic is congested. To add to the steering assistance, most of the power-assisted steering systems incorporate hydraulic servo mechanisms by which the hydraulic power available in the systems is reinforced or multiplied. Such servo mechanisms are required, in addition to their inherent performances, to provide the vehicle driver a feel of the road" or reaction from the steered front wheels and to enable him to perceive at the steering wheel the tendency of the vehicle to straighten out from a turn of the front tires. During driving of the motor' vehicles at relatively high speeds, moreover, it is important that the reaction be augmented and stabilized to assure safety of driving. The servo mechanisms are thus provided with hydraulic reaction chambers by which the force resulting from the hydraulic power for the steering is returned to the steering wheel and control devices by which the hydraulic power entering the reaction chambers is varied in accordance with the vehicle driving speeds. A typical example of such control devices includes a steeringcontrol valve which is adapted to respond to steering resistance and control the pressure of the fluid from an engine driven pump in accordance with the steering resistance and a speed-sensitive control unit which is adapted to produce an electric signal representingthe vehicle driving speed. The fluid pressure which has been controlled by the steering-control valve is thus further controlled in accordance with the vehicle speed before it is directed to the hydraulic reaction chamber. When, for instance, the vehicle is being driven at a relatively high speed and a relatively great steering resistance is encountered in the steering system, then the fluid pressure is controlled to develop a relatively high hydraulic pressure and vice versa. The fluid pressure to be directed to the reaction chamber is, therefore, not only varied with the steering resistance but regulated in a manner to require more physical effort applied to the steering wheel during high-speed cruising and less physical effort during low-speed driving.

Limitations are, however, encountered in the performances of the prior art hydraulic servo mechanisms of the above outlined nature. Foremost of such limitations is the fact that the fluid pressure to be directed to the hydraulic reaction chamber can not exceed the level of the pressure of the fluid primarily controlled by the steering-control valve. This invites lack of stability in steering operations because only limited reactions are imparted to the steering wheel especially during high-speed driving. Another limitation results from the fact that the power assistance achieved by the fluid pressure of the pump delivery as controlled by the steering-control valve is practically negligible when no, or only slight, physical effort is applied to the steering wheel as during normal, straight-highway driving, al-

though such steering-assistance fluid pressure rises as the steering resistance increases. This, again, results in lack of stability during high-speed driving on a straight road.

These limitations will be more or less raised if the pressure acting areas in the hydraulic reaction chamber are considerably expanded as practised in the conventional art. This, however, is reflected by the disproportionately large-sized construction of the reaction chamber and, as such, has not been fully accepted especially in the steering servo mechanisms of the integral type in which the hydraulic reaction chamber and steeringcontrol valve are combined integrally with a steering power cylinder. The increased pressure acting areas in the reaction chamber further creates a difficulty in that an unduely high fluid pressure is produced by the steering-control valve at the initial stage of the steering operation. A considerably increased physical effort should therefore be exerted on the steering wheel during lowand intermediate-speed driving, so that the intent of the power-assisted steering is almost jeopardized. Drivers feeling of uncomfortableness at a transitional moment from the manual (or unassisted) to assisted conditions may also be pointed out.

The present invention contemplates provision of an improved hydraulic-power control device for use in the power-assisted steering system having the reaction chamber whereby the above noted drawbacks inherent in the prior art counterparts are eliminated.

It is, therefore, an object of the present invention to provide an improved hydraulic-power control device which is adapted to make steering more effortless and reliable under various driving conditions of the motor vehicle.

It is another object of the invention to provide an improved hydraulic-power control device providing ease and reliability of power-assisted steering for parking or during congested traffic conditions and for high-speed driving of the motor vehicle.

It is still another object of the invention to provide an improved hydraulic-power control device by which proper reactions are imparted to the steering wheel under any driving condition of the motor vehicle so as to enable the driver to steer the vehicle with sufficient stability.

It is still another object of the invention to provide an improved hydraulic-power control device which is ca pable of supplying to the reaction chamber a fluid pressure which may be higher, under predetermined conditions, than the fluid pressure developed to provide the steering-assistance power.

It is still another object of the invention to provide an improved hydraulic-power control device by which completely stabilized power-assisted steering is achieved even during straight-highway driving of the motor vehicle at elevated speeds.

It is still another object of the invention to provide an improved hydraulic-power control device .which is adapted to be compatible with the reaction chamber having the construction of usual size. Thus, the device according to the present invention is well compatible with the steering servo mechanisms of the integral type.

It is still another yet important object of the invention to provide a hydraulic-power control device which is operable to boost or multiply an input fluid pressure to practically unlimited, proper levels depending not only upon the steering resistance as encountered in the steering system but upon the speed of the motor vehicle under any driving condition.

It is still another object of the invention to provide an improved hydraulic-power control device having a simple and compact construction which is easy and economical to manufacture on a commercial basis.

These and other objects of the present invention are generally accomplished in a construction comprising a steering-control valve unit which is responsive to steering resistance from front wheels of the motor vehicle and which is hydraulically connected to a constantdelivery pump so that the fluid pressure of the pump delivery is controlled to develop a steering-assistance fluid pressure in accordance with the steering resistance, speed-sensitive control means which is responsive to driving speed 'of the motor vehicle and producing an electric signal representative of the vehicle driving speed, a pressure control valve unit responsive to the steering-assistance fluid pressure and to the electric signal supplied from the speed-sensitive control means so as to produce a control signal which is related to the steering-assistance fluid pressure and the electric signal, and a booster unit which is hydraulically connected to the pump, reaction chamber and steering-control valve unit for varying a fluid pressure to be directed to the reaction chamber in accordance with the control signal supplied from the pressure control valve unit.

The pressure control valve unit may be connected to the booster and steering-control valve unit either hydraulically or hydromechanically. Where, thus, the pressure control valve unit is hydraulically connected to the booster and steering-control valve unit, it supplies to the booster a signal fluid pressure as the aforementioned control signal in accordance with the steering-assistance fluid pressure and the electric signal supplied from the speed-sensitive control means. In this instance, the booster unit may be constructed in a manner to comprise a balanced valve having opposed, larger and smaller working faces and a pressure-control passageway for providing interruptible fluid communication between the pump and steering-control valve unit through an adjustable flow restriction which is adjustable by movement of the balanced valve due to a difference between thepressures acting upon the working faces thereof. The larger working face of the balanced valve is subjected to the signal fluid pressure supplied from the pressure control valve unit while the smaller working face is subjected to the pressure of the pump delivery. The adjustable flow restriction in the pressure control passageway is constricted or even closed when the balanced valve is moved by the aforesaid signal fluid pressure against the pump delivery pressure acting on the smaller working face of the balanced valve, so as to give rise to an increase in the pump delivery pressure. It is preferable that the booster unit is further provided with a valved passageway leading at one end from a pump side of the pressure-control passageway and opened at the other to the outside so as to drain off the pump delivery when the pump delivery pressure is increased beyond a predetermined level. A similar valved passage way may also be formed in the booster unit, leading at one end from a steering-control valve unit side of the pressure-control passageway and opened at the other to the outside for relieving the steering-assistance fluid pressure as soon as the pressure is increased beyond a predetermined level.

The pressure control valve unit thus supplying the signal fluid pressure to the booster may comprise a first valve member which is operable to cut off the delivery of the steering-assistance fluid pressure from the steering-control valve unit to the booster unit when actuated, a second valve member operable to relieve the steering-assistance fluid pressure at a limited rate when actuated, and first and second electromagnetic actuating means connected to the aforesaid speed-sensitive control means and respectively associated with the first and second valve members for actuating the valve members when energized.

The first electromagnetic actuating means is energized when the electric signal from the speed-sensitive control means is representative of a vehicle driving speed which is lower than a predetermined relatively low level. The second electromagnetic actuating means, on the other hand, is energized when the electric signal from the speed-sensitive control means is indicative of a vehicle driving speed which is higher than a relatively high level. Where the pressure control valve unit is constructed in this manner, it may also comprise a first passageway which is hydraulically connected at one end to the steering-control valve unit through a flow restriction and at the other to the booster unit and a second passageway leading at one end from the first passageway through a flow restriction and opened at the other to the outside. The first and second valve members are respectively positioned in a manner to shut-off the first and second passageways posterior to the associated flow restrictions therein when the valve members are actuated.

Where an arrangement is preferred in which the pressure control valve unit is hydromechanically connected to the booster unit and steering-control valve unit, the pressure control valve unit may be so constructed as to supply to the booster unit amechanical signal as the above mentioned control signal in accordance with the steering-assistance fluid pressure and the electric signal supplied from the speed-sensitive control means. The booster unit may then comprise first and second balanced valves each having opposed, smaller and larger working faces, a first passageway for providing fluid communication between an input side of the first balanced valve and an output side of the second balanced valve through output and input sides of the first and second balanced valves respectively and a second passageway for providing interruptible fluid communication between the output sides of the first and second balanced valves. The smaller working faces of the first and second balanced valves are subjected to respective input pressures to the valves and the larger working faces are subjected to respective output pressures from the valves. The input pressure to the first balanced valve and the output pressure from the second balanced valve correspond respectively to the pressure of the pump delivery and the steering-assistance fluid pressure. The first passageway is provided with first and second adjustable flow restrictions intervening between the input and output sides of the first and second balanced valves, respectively. These flow restrictions are adjustable by movements respectively of the first and secondbalanced valves due to differences between the input and. output pressures acting upon the respective valves. Each of the output pressures from the first and second balanced valves is thus varied in accordance with the mechanical signal which is supplied by the pressure control valve unit as previously mentioned.

This pressure control valve unit may be made up of first and second electromagnetic actuating means which are adapted to be energized in accordance with the electric signal from the speed-sensitive control means and first and second fluid flow control valves which are operatively connected to the first and second electromagnetic actuating means, respectively, and each of which is actuated when the associated actuat ing means is energized. The first and second fluid flow control valves are positioned in association with the output sides of the first and second balanced valves of the booster unit so that the second passageway in the unit is controlled by each of the flow control valve thereby to vary each of the output pressures from the first and second balanced valves.

Other features and preferred embodiments of the hydraulic-power control device having the above described general nature will be apparent from the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram showing a general construction of a prior art hydraulic-power control device for use with a steering servo mechanism having a hydraulic reaction chamber;

FIG. 2 is a block diagram which schematically illustrates basic construction principle governing various preferred embodiments of the present invention;

FIG. 3 is a view showing a first preferred embodiment of the device according to the present invention having a booster unit illustrated by its section and associated means and units shown in a blocltform;

FIG. 4 is a graph showing an example of the variations in the pump delivery pressure in terms of the steering-assistance pressure at two different driving speeds of the motor vehicle as attainable where the device shown in FIG. 3 is placed on use;

FIG. 5 is a graph showing an example of the relationship between the steering resistance and the steering reaction at two different vehicle driving speeds as attainable by the device shown in FIG. 3.

FIG. 6 is a sectional view, partly in a block form and not showing a speed detector, of a second preferred embodiment of the device according to the present invention;

FIG. 7 is a graph indicating an example of the variation in the steering reaction in terms of the steering resistance and different driving speeds of the motor vehicle as attainable by the use of the device shown in FIGS. 6 and 8;

FIG. 8 is a block diagram illustrating a preferred electric arrangement of the speed-sensitive control means forming part of the device which is illustrated in FIG.

FIG. 9 is a block diagram showing a third preferred embodiment of the device according to the present invention;

FIG. 10 is a sectional view showing, in detail, a preferred construction of the booster and pressure control valve units forming part of the device illustrated in FIG.

FIG. 11 is a graph indicating an example of the variation in the steering reaction in terms of the steering resistance and different driving speeds of the motor vehicle as attainable by the use of the device shown in FIGS. 9, and 10;

FIG. 12 is a view similar to FIG. 10 but now shows a modification of the device shown therein.

FIG. 13 is a block diagram of a preferred electrical arrangement of the speed-sensitive means of the device shown in FIG. 9;

Reference is first made to FIG. 1 to clearly bring out the previously discussed drawbacks of the prior art hydraulic-power control device for the servo mechanisms of the power-assisted steering systems.

As illustrated in FIG. 1, the hydraulic-power control device of prior art hydraulically interconnects a constant delivery pump A (usually driven by a vehicle power plant) and a hydraulic reaction chamber B which forms part of the servo mechanism, not shown. The control device includes a steering-control valve C which is responsive to the steering resistance or load created in the steering system and a vehicle-speed detector D which is responsive to the driving speed of the motor vehicle on which the steering system is installed. The steering-control valve C is connected by a hydraulic line to the constant-delivery pump A and controls the pressure of the pump delivery to develop a steeringassistance fluid pressure Pa. This steering-assistance fluid pressure Pa is directed to a pressure control valve F to which the steering-control valve C is connected by a hydraulic line. The vehicle-speed detector D, on the other hand, produces an electric signal which is generally proportional to the detected vehicle speed and supplies the signal to a module E. This module is constructed to convert and amplify the input signal into an appropriate control signal which is predetermined depending upon desired operation characteristics of the control device. The module F is electrically connected to the pressure control valve E so that the steeringassistance fluid pressure Pa supplied to the control valve is further modified in accordance with the control signal from the module, thereby to develop a finally controlled pressure Fc which is to be directed to the reaction chamber B through a hydraulic line. A reactive force thus developedin the reaction chamber B is returned to the steering wheel so as to provide the driver the feeling of reaction from the steering system. The control device is designed to increase such steering reaction during high-speed driving of the motor vehicle and to decrease the reaction during low-speed driving so as to assure stable and smooth steering operations under various driving conditions of the motor vehicle. In the shown hydraulic-power control device of prior art, however, it is impossible that the fluid pressure Pc delivered from the pressure control valve F and directed to the reaction chamber B is increased beyond the steering-assistance pressure Pa. The fluid pressure Pc only becomes equal to the pressure Pa even when it is increased to a maximum. Moreover, the steeringassistance fluid pressure Pa is kept at an extremely low level when no, or only negligible, steering effort is applied to the steering system such as during straighthighway driving. These result in lack of stability of steering operation especially during high-speed driving of the motor vehicle, as previously pointed out.

A general constructional arrangement of the hydraulic-power control device according to the present invention which is free from the above noted drawbacks is now illustrated in a block form in FIG. 2. Similarly to the prior art hydraulic'power control device shown in FIG. 1, the device according to the invention includes a steering-control valve unit 10, a vehicle speed detector 11, a module or electric control circuit 12, and a pressure control valve unit 13. All these may be constructed in an essentially similar manner to their counter-parts of the device prior art, unless otherwise specifled in the description to follow. The steering-control valve unit 10, for instance, may be constructed in a manner to develop a fluid pressure which is generally proportional to the steering resistance appearing in the steering system. The vehicle speed detector 11, on the other hand, may be constructed in a manner to detect the revolution speed of either the vehicle wheel or the propeller shaft so as to produce a signal voltage proportional to the detected revolution speed. The construction of the pressure control valve unit 13 may be such that it has an adjustable orifice with its working area adjusted by a solenoid device which may be energized from the electric control circuit 12 or any other electrically operating speed-sensitive control means.

Different from the device shown in FIG. 1, the hydraulic-power control device according to the present invention is further provided with a booster unit 14 which is adapted to multiply the steering-assistance fluid pressure Pa developed by the steering-control valve unit 10. The booster is hydraulically connected on one side to the constant-delivery pump A and reaction chamber B of the steering servo mechanism, not shown, and on the other side to the steering-control valve unit 10,.The pressure control valve unit 13 is either hydraulically or hydromechanically connected on one side to the steering-control valve unit and on the other to the booster unit 14 while being electrically connected to the electric control circuit 12. I

The detailed construction of the booster unit 14 which forms part of the first preferred embodiment of the hydraulic-power control device according to the present invention is now illustrated in FIG. 3. The booster unit 14 as shown has a housing structure 15 having formed therein a generally cylindrical bore 16. This cylindrical bore 16 communicates with the pump A through an inlet port 17 and with the hydraulic reaction chamber B through an outlet port 18 wich leads from the inlet port 17. The cylindrical bore 16 further communicates with the steering-control valve unit 10 and an input side of the pressure control valve unit 13 through a port 19 and with an output side of the pressure control valve unit 13 through an inlet port 20. Thus, a pump-delivery pressure P,, obtains in the ports 17 and 18, a steering-assistance fluid pressure Pa developed by the steering-control valve unit 10 obtains in the port 19, and a signal fluid pressure Ps developed by the pressure control valve unit 13 obtains in the port- 20. A directional balanced valve 21 is axially slidably received in the cylindrical bore 16. This balanced or piston valve 21 has a larger-diameter land 22 formed at one end of the valve adjacent the inlet port and spaced smaller-diameter lands 23 and 24 defining therebetween an annular groove 25. One smallerdiameter land 23 projects from an inner face of the larger-diameter land 22 while the other smallerdiameter land 24 is located at an end of the valve 21 opposite to the larger-diameter land 22. The smallerdiameter lands 23 and 24 have working faces of equal areas so that the fluid pressure acting thereupon results in no mechanical displacement of the valve 21 in its entirety. The larger-diameter land 22, on the other hand, has its inner working face 22a subjected to the pump delivery pressure P, directed to an input side of the valve 21 through the inlet port 17 and its outer working face 22b subjected to the signal fluid pressure Ps directed into the bore 16 through the inlet port 20 from the pressure control valve "unit 13, as illustrated. The outer'working face 22b of the land 22 thus defines a signal fluid chamber 26 with an end wall of the cylindrical bore 16. The inner working face 22a is smaller than the outer working face 22b and, as such, they are herein referred to as smaller and larger working faces, respectively, to provide simplicity of definition in the appended claims.

The annular groove 25 defined between the smallerdiameter lands 23 and 24 is on one side in constant communication with the port 19 leading to the steering-control valve unit 10 and on the other side in interruptible communication with the inlet port 17 and accordingly to the outlet port 18 through an adjustable flow restriction or orifice 27 which is defined by cooperating circumferential edges of a stepped portion of the cylindrical bore 16 and the smaller-diameter land 23. The valve 21 is thus axially movable when there is a difference between the pressures acting upon the smaller and larger working faces 22a and 22b, respectively, and rests in a balanced axial position when such pressures areequalized.

In order that the fluid 'pressures P, and P, be prevented from being excessively increased for one reason or another, the booster unit 14 may be further provided with a suitable pressure relief arrangementpFor this purpose, bypass passages 28 and 29 are formed in the housing structure'15 in association with the ports 17 and 19, respectively. The bypass passage 28 leads at one end from the inlet port 17 and is opened at the other end to an end portion 30 of the bore 16 while the bypass passage 29 leads at one end from the port 29 and is opened at the other end tothe end portion 30 of the bore The bypass passage 28 communicates with the port 19 through a passage 28a through the bore 16, as shown. Spring-loaded pressure relief valves 31 and 32 are interposed between ends of the bypass passages 28 and 29, respectively, in a manner that each of the valves 31 and 32 is moved to a position to pass the fluid in each of the ports 17 and 19 to the end portion 30 of the bore 16 if and when the pressure of the fluid rises in excess of a predetermined level. These valves 31 and 32 are herein shown as poppet valves which are biased to close the bypass passages 28 and 29 by means of preloaded springs 33 and 34, respectively. The end portion 30 of the bore 16 is opened to the outside or, as illustrated, led to the pump A through a port 35 formed in the housing structure 15 and a reservoir 36 so that the excess fluid passed to the end portion 30 of the bore 16 is returned to the pump. This reservoir 36 may be connected also to the steering-control valve unit 10 and pressure control valve unit 13, where desired, though not shown.

When, in operation, the motor vehicle is driven at an increasing speed and with an increasing steering resistance encountered, then the pressure control valve unit 13 delivers an increasing signal pressure P,. This causes the fluid pressure in the fluid chamber to increase so that the balanced valve 21 is moved away from the port 20 against the pressure P, acting on the smaller working face 22a of the land 22. The working area of the adjustable orifice 27 is consequently reduced by movement of the intermediate land 23 toward the circumferential edge of the stepped portion of the cylindrical bore 16. If, now, the areas of the smaller and larger working faces 22a and 22b of the larger-diameter land 22 are donoted by S, and 5,, respectively, where S, 8,, then the balanced valve 21 is moved leftwardly of the drawing by a force represented by (P, X 8,) overcoming an opposing force represented by (P, X 8,). The orifice 27 now being constricted by the movement of the balanced valve 21, the flow of the fluid from the port 17 to the port 19 is subjected to an increased resistance so that the pump delivery pressure P, rises with the consequent increase in the force (P, X S, )acting upon the smaller working face 22a of the land 22. The valve 21 is accordingly moved in a reverse direction, viz., toward the port 20 until equilibrium is achieved between the two opposed forces on the smaller and larger working faces of the land 22. Under this condition, the pump delivery pressure P, is given by an equation:

(P =8, /S, P,

This shows that the pump delivery pressure P, is multiplied to the signal fluid pressure P, times S,,S, which is greater than 1. It is thus noticed that the fluid pressure P, to be directed to the reaction chamber B is far greater than the pressure which is attained in the prior art hydraulic-power control devices in which the signal fluid pressure P, in the device according to the present invention is directly fed to the reaction chamber B. If, for instance, the. pressure control valve unit 13 is so constructed as to develop pressure P, =S,/S, during low-speed driving of the motor vehicle and pressure P, P, during high-speed driving, then the pump delivery pressure P, to be directed to the reaction chamber B will be expressed, for the low-speed driving condition, as: P,= (S, /S, P, S,/S S IS, P, =P,

and, for the high-speed driving condition, as: P, (S, P' s (S8 /SP) a- From this it is understood that the physical effort on the steering wheel is relieved of the driver as in the case of the prior art power-assisted steering systems during the low-speed driving condition and increased S ,/S, fold as compared with the case of the prior art steering systems during the high-speed driving condition.

When, now, the pump delivery pressure P, further rises to such an extent that the'difference between the fluid pressures P, and P, reaches a predetermined level, the pressure relief valve 31 opens the bypass passage 28 so that the excess fluid pressure is relieved to the reservoir 36 through the port 35, whereby an exces sive increase in the steering reaction or application of an overload on the pump P can be avoided. When, on the other hand, the steering-assistance fluid pressure P, developed by the steering-control valve unit is excessively increased to reach a predetermined level, then the pressure relief valve 32 opens the bypass passage 29 with the result that the excess fluid in the port 19 is admitted to the port 35. The steering-assistance fluid pressure P, is in this manner maintained under the predetermined level.

FIGS. 4 now indicates an example of the relationships between the steering assistance fluid pressure P, (on the axis of abscissa) and the pump delivery pressure P, (on the axis of ordinate) to be directed to the reaction chamber B, as attainable in the hydraulicpower control device shown in FIG. 3. The plot joining points 0 and Y represent such relationship achieved during low-speed driving of the motor vehicle while the plot joining points 0, X and Y stands for the relationship achieved during high-speed driving. The plot O-Y, is thus in agreement with the variation in the steeringassistance fluid pressure P, itself. Under the high-speed driving condition of the motor vehicle, as observed from the plot O-X-Y,, the pump delivery pressure P, increases abruptly before the steering-assistance fluid pressure P, reaches a predetermined point as indicated by line segment O-X. Once the point X is reached by the pump delivery pressure P, so that the difference between the pressure P, and P, is P,,, the increase in the pump delivery pressure P, slows down as indicated by line segment X-Y Where desired, the grade of the line segment X-Y may be made zero so that the pump delivery pressure P, is maintained at level X after the point X has been reached or, otherwise, the line segment X-Y, may have a grade which is substantially equal to the grade of the plot 0-Y, for the low-speed driving condition so that the pump delivery pressure P, is increased at the same rate as the rate of increase of the steering assistance fluid pressure P,. Such characteristics of the pump delivery pressure P, during the conditions corresponding to the line segment X-Y, may be determined through selection of the geometry of the bypass passage 28 and pressured relief valve 31 (FIG. 3). If, thus, an arrangement is made so that the pressure acting areas on the input and output sides of the pressure relief valve 31 are denoted by A, and A,, respectively, then the relation between the values AP, and AP, will be given by so that From this it is apparent that the grade of the plot X-Y, can be varied arbitarily through selection of the geometry of the bypass passage 28 and pressure relief valve 31.

It may be added that the pump delivery pressure P, is not increased beyond the level Y for the low-speed driving condition or the level Y, for the high-speed driving condition because the steering-assistance fluid pressure P, can not increase beyond its maximum level P,, at which the pressure relief valve 32 opens the bypass passage 29 to drain off the fluid in the port 19.

FIG. 5 illustrates an example of the relationship between the steering resistance (on the axis of abscissa) and the steering reaction (on the axis of ordinate), as attainable by the hydraulic-power control device shown in FIG. 3. The points X, Y and Y as indicated in FIG. 5 are in agreement with their counterparts in the plots of FIG. 4. Before the steering resistance reaches a certain level, the steering reaction increases at a rate similar to the rate of its increase in the case of the manual or unassisted steering operation, as indicated by the plot 01,, for the low-speed driving condition and the plot O-Z, for the high-speed driving condition. This is because of the fact that, since the steering control valve unit is subject to a certain pressure or preload at an initial stage of the steering operation, the servo mechanism is kept at rest until the steering resistance equals the steering reaction. When, thus, the steering resistance increases and accordingly the servo mechanism becomes operative, steering effort is developed in the reaction chamber so that the powerassisted steering operation is commenced once the physical effort for the manual steering exceeds the effort applied by the reaction chamber. When the steering resistance increases to reach the upper limit P of the steering-assistance fluid pressure, then the pump delivery pressure P no longer increases and accordingly the steering effort applied by the servo mechanism ceases increasing. If, therefore, the steering resistance still increases, the power-assisted steering effort rises from point Y, or Y at the same rate as in the case of the manual steering operation.

Since, in this instance, the steering-control valve unit is subject to the initial pressure or preload even during the straight-ahead condition of the steering system, the pump delivery pressure P, increases as the vehicle is driven at an increasing speed. It therefore follows that, during high-speed driving on a straight road, the powerassisted steering effort starts to be applied by the reaction chamber at point Z which is higher than point Z, for the low-speed driving condition. A controlled reaction is in this manner imparted to the steering wheel during straight-highway driving, providing the driver with a feeling of assured steering.

Another preferred embodiment of the hydraulicpower control device in accordance with the present invention is illustrated in F IG. 6. The hydraulic circuit arrangement of the device shown herein is essentially similar to that of the device illustrated in FIG. 3 and, thus, discussion which has been given on the arrangement of FIG. 2 will be applicable thereto as it is. FIG. 6, however, shows a representative example of the constructions of the steering-control valve unit and reaction chamber B for reference purposes. As seen in the lower portion of FIG. 6, the steering-control valve unit 10 includes a spool valve 37 interposed between a steering gear drop arm or an idler arm-38 and a double- .acting power cylinder 39 having a piston 40. The valve unit 10 is thus shown as integral with the power cylinder 39, the shown servo mechanism therefore being of the integral type which was mentioned at the outset of this description. The spool valve 37 is centralized by springs 41 which are accommodated within the reaction chamber B formed in the valve body. The pump delivery pressure P, is directed to this reaction chamber through a port 42 while the steering-assistance fluid pressure P, is directed to annular grooves in the spool valve 37 through a port 43. This pressure P,, is then directed to pressure acting chambers, not numbered, in the power cylinder 39 through passages 44 and 44, as illustrated. The operation of the servo mechanism having the above noted construction is well known in the art and, in addition, such construction is merely shown merely by way of example, no detailed description will be herein incorporated in connection with the shown mechanism.

Now, the hydraulic-power control device shown in FIG. 6 has incorporated therein an improvement which is directed to the pressure control valve unit 13. The pressure control valve unit 13 thus includes a valve housing 45 in which passages 46 and 47 are formed. One passage 46 leads to the output and input sides of the booster unit 14 through its inlet and outlet ports 46a and 46b, respectively. Between these inlet and outlet ports 46a and 46b is interposed a flow restriction or orifice 48 so as to cause a drop in the pressure of the fluid passing therethrough. Where the pressure control valve unit 13 of the shown construction is used in combination with the booster unit which is specifically constructed as illustrated in FIG. 3, these output and input sides will correspond to the ports 19 and 20, respectively. The steering assistance pressure P is thus directed to the passage 46 anterior to the orifice 48 and the signal fluid pressure P, is delivered from the passage posterior to the orifice 48 during operation, as will be discussed later. The other passage 47 communicates at one end to the passage 46 downstream of the orifice 46 through a flow restriction or orifice 49 and opened at the other end to the outside or otherwise led to the reservoir, not shown, to return the fluid to the pump A. A pair of spring-loaded poppet valves 50 and 51 are mounted in the valve housing 45 in a manner to control the flows of fluid in the passages 46 and 47, respectively. One poppet valve 50 projects into the passage 48 posterior to the orifice 48 while the other poppet valve 51 projects into the passage 47 posterior to the orifice 49. These poppet valves 50 and 51 are respectively biased by preloaded springs 52 and 53 to fully open the passages 46 and 47. These springs 52 and 53 are shown as seated on spring seats 54 and 55, respectively. which are attached to ends of the valves. A pair of plungercd solenoid devices 56 and 57 are mounted on the valve housing 45 in association with the poppet valves 50 and 51. These solenoid devices 56 and 57 include plungers 58 and 59, respectively, which are integral with or securely connected to armatures of the solenoid devices. These plungers 58 and 59 are respectively bear against the poppet valve 50 and 51 through the spring seats 54 and 55 and, when the solenoid devices 56 and 57 are de-energized, impart no driving actions to the associated poppet valves, as illustrated.

The solenoid devices 56 and 57 are connected to the speed-sensitive control which is adapted to detect the driving speed of the motor vehicle and to produce an electric signal representing the detected vehicle speed. Such control means may include the vehicle speed detector 11 and electric control circuit 12 shown in FIG. 2.

When both of the solenoid devices 56 and 57 remain concurrently de-energized, then the poppet valves 50 and 51 are held in retracted positions to fully open the passages 46 and 47 by the actions of the preloaded springs 52 and 53, respectively. As a consequence, the steering-assistance fluid pressure P, entering the passage 46 is regulated to the signal fluid pressure P, as it is reduced at the orifice 48 and partly discharged out of the passage 47 at a rate limited by the orifice 49 therein. The signal fluid pressure P, to be returned to the booster unit 14 is therefore given, if d and d denote respective diameters of the orifices 48 and 49, by

If, then, the solenoid device 56 is energized with the other solenoid device 57 kept de-energized, the plunger 58 protrudes so as to move the poppet valve 50 against the action of the spring 52 to an operative position to close the passage 46 anterior to its outlet port 46b and to the passage 47. The fluid pressure entering the passage 46 is completely cut off so that no fluid pressure obtains at the outlet port 46b of the passage 46. The signal fluid pressure P, is accordingly given by If, conversely, the solenoid device 57 is energized with the solenoid device 56 de-energized, then the plunger 59 protrudes so as to move the poppet valve 51 against the action of the spring 53 to an operative position to close the passage 47. Since, in this condition, the passage 46 is fully open, the steering-assistance fluid pressure P., is discharged from the outlet port without reduction so that the following relation now holds:

The operative and inoperative conditions of the solenoid devices 56 and 57 are scheduled by properly programming the speed-sensitive control means in a manner to approximately follow the vehicle driving speeds, as previously mentioned. Table I shows an example of such schedules, in which the indication ON refers to an energized condition of the solenoid device and the indication OFF to a de-energized condition of the divece, and in which the characters V and V, are different driving speeds of the motor vehicle where v v,.

TABLE I Vehicle speed Solenoid Solenoid Up to V ON OFF V, to V, OFF OFF Over V OFF ON The operation of the hydraulic-power control device having the pressure control valve unit 13 shown in FIG. 6 will now be described with reference to the above Table I and FIG. 7. FIG. 7 shows an example of the relationship between the steering resistance (on the axis of abscissa) and the steering reaction (on the axis of ordinate) as attainable by the use of the device shown in FIG. 6.

When the motor vehicle is being driven at a speed lower than V the solenoid device 56 is energized and the solenoid device 57 is de-energized so that the signal fluid pressure P, is dictated by Eq. 2. Thus, in the absence of the signal fluid pressure, the booster unit 14 is kept inoperative and hence P z P The steering reaction characteristics achieved in this condition is simi lar to those available by the conventional powerassisted steering system, as indicated by plot a in FIG. 7.

As the vehicle speed increases beyond the level V; but is lower than the level V then the solenoids 56 and 57 are concurrently de-energized so that Eq. 1 now applies. By the aid of the signal fluid pressure P, as indicated by Eq. 1, the booster unit 14 develops the pump delivery pressure P, which is expressed as P, K-P, where K represents the pressure multiplification factor achieved by the booster unit. From this and in consideration of the relation given by Eq. 1, the pump delivery pressure P,. may be written in the form P, K /(l 1 2) a It therefore follows that the ratio of the fluid pressure to be directed to the reaction chamber B vs. the steering-assistance fluid pressure given as follows:

P /P (if/dz). This brings out that the steering effort is multiplied by K/( I d ld as compared with the condition in which the motor vehicle is driven at a speed lower than the level V,. The steering reaction characteristics thus achieved is indicated by plot b in FIG. 7.

If the vehicle'driving speed is further increased beyond the level V,, then the solenoid 57 alone is energized so that, from Eq. 3 the signal fluid pressure developed by the pressure control valve unit of FIG. 6 is given by From this it follows that the fluid pressure P to be directed to the reaction chamber becomes P, K'P, K'P

so that P,,/P,, K. This will mean that the steering effort required in this condition is multiplied by K as compared with the conventonal power-assisted steering system. The steering reaction characteristics is thus analogous to that exhibited during manual steering, as ascertained by plot 0 of FIG. 7.

It has been assumed that the vehicle speed is increased through V and V in the foregoing description but essentially the same results will be achieved if the vehicle speed decreases through V: and V with the solenoid devices 56 and 57 energized and de-energized as scheduled in Table I.

If, furthermore, the geometry of the poppet valves 50 and 51, performance characteristics of the springs 52 and 53 and/or thrusts by the plungers of the solenoid devices 56 and 57 are suitably selected, the pressure control valve unit 13 having the construction shown in FIG. 6 may be modifiedin a manner that the poppet valves 50 and 51 hold the previously set positions whether the solenoid devices 56 and 57 are energized or de-energized if, the pressure differences across the poppet valves are greater than predetermined levels. In this instance, the poppet valves may be moved to change the signal fluid pressure P, when, for example, the steering system resumes the straight-ahead condition with the consequent reduction in the steeringassistance pressure P,,. The poppet valves 50 and 51 in such arrangement are thus best bestowed with selfmaintaining characteristics which will contribute to preventing unexpected or abrupt variation in the steering reaction during steering operation so that the steering reaction can be varied only during straight'ahead operation. As an alternative to such self-maintaining characteristics provided on the poppet valves, the poppet valves may be controlled to achieve the same results through suitable electrical arrangement which may contain means to detect the steering angle in the steering system.

Turning back to FIG. 7, the plot e indicates the steering reaction characteristics resulting from the operation of the booster unit 14, viz., from the motion of a pressure relief valve which is mounted in the booster unit in order to relieve the pump delivery pressure when the pressure increases beyond a predetermined level. This pressure relief valve may be constructed, by way of example, as the valve 31 in the booster unit 14 shown in FIG. 3. The plot f, on the other hand, the steering reaction characteristics which is achieved by the use of a pressure relief valve which may be mounted in the booster unit so as to relieve the steeringassistance pressure P in the event the pressure increases to a predetermined level. This pressure relief valve may-be constructed as the valve 32 in the embodiment shown in FIG. 3.

The hydraulic-power control device shown in FIG. 6 has advantages which are summarized as follows:

a. Lower production cost and higher reliability than the prior art hydraulic-power control devices using servo valves.

b. Safety driving of the motor vehicle and assured feeling in steering because of the steering reaction changed only during straight-ahead condition.

c. Pump capacity saved because the signal fluid pressure is cut off during low-speed driving.

d. Limited power consumption by the solenoid devices because they are de-energized under the intermediate-speed driving condition which is set up most frequently by the motor vehicle.

e. F ail-safe characteristics by which the pressure control vaves assume the conditions of the intermediate-speed driving in the event the unit or associated parts and elements fail.

FIG. 8 now illustrates a preferred form of speedsensitive control means which is adapted to selectively energize and de-energize the solenoid devices of the pressure control valve unit of the embodiment shown in FIG. 6 in accordance with the driving speed of the motor vehicle.

Referring to FIG. 8, the speed-sensitive control means includes a vehicle-speed detector 60 which detects the revolution speed of the vehicle wheel or wheels or of the propeller shaft of the vehicle driveline and produces a signal voltage which is proportional to the detected vehicle speed. This signal voltage is converted into an A.C. voltage by a D.C.-to-A.C. converter 61 and the thus obtained A.C. voltage is supplied to a switch circuit 62. The switch circuit 62 is made up of a pair of Schmidt trigger circuit 63 and 63' having trigger levels corresponding to the vehicle speeds V and V respectively. One Schmidt trigger circuit 63 is connected to a NOT-gate circuit 64 which, in turn, is connected to the solenoid cell of the solenoid device 56 through an amplifier 65. The other Schmidt trigger circuit 63' is connected to the solenoid device 57 through an amplifier 65'.

When, thus, the vehicle-speed detector 60 detects a vehicle driving speed which is lower than the level of V then both of the Schmidt trigger circuits 63 and 63 are inoperative. The NOT-gate circuit therefore produces an output to energize the solenoid device 56 while, in the absence of an output from the Schmidt trigger circuit 63, the other solenoid device 57 is deenergized'. This is in agreement with the schedules of Table I.

If the detector 60 detects a vehicle speed which is intermediate between V, and V then the Schmidt trigger circuit 63 becomes operative so that the NOT-gate does not produce the output while the Schmidt trigger circuit 63' is kept inoperative. This established the condition in which the two solenoid devices 56 and 57 are de-energized, as indicated in Table I.

When the detector 60 detects a vehicle speed which is higher than the level of V then both of the Schmidt trigger circuits 63 and 63' become operative so that the solenoid device 56 is de-energized and the solenoid device 57 energized, as scheduled in Table I.

In order to provide the previously mentionedselfmaintaining characteristics of the pressure control valve unit, the circuit arrangement shown in FIG. 8

may be further provided with a hysteresis element or circuit, if desired, g

A third preferred embodiment of the hydraulic power controldevice in accordance with the present invention will now be described with reference to FIGS. 9 and 10. The embodiment herein shown is characterized in that the pressure control valve unit is hydromechanically connected to the booster unit. different from the previously described embodiments in which they are only hydraulically associated with each other. Further different from the prior embodiments, the third embodiment is adapted to control the booster in accordance with four different vehicle speed ranges. For this purpose, the booster unit comprises two mutually associated parts which are herein referred to as first and second booster sections 66 and 67, respectively, as shown in block form in FIG. 9. These first and second booster sections 66 and 67 are connected in series with the steering control valve unit 10 and reaction chamber B. The booster sections 66 and 67 are associated respectively with first and second pressure control valve sections 68 and 69, respectively, which, as combined, constitute the pressure control valve unit of the hydraulic-power control device according to the present invention. These first and second pressure control valve sections include first and second solenoid devices 70 and 71, respectively, which are connected to suitable speed-sensitive control means adapted to produce an electric signal in accordance with the motor vehicle driving speed. The detailed constructions of the booster sections 66 and 67 and pressure control valve sections 68 and 69 are illustrated in FIG. 10.

Referring to FIG. 10, the booster unit as shown comprises a housing structure 72 having parallel, generally cylindrical, first and second bores 73 and 74, respectively. The first bore 73 communicates on its input side with the pump A through an inlet port 75 and with the reaction chamber B through an outlet port 76 which leads from the inlet port 75. The second bore 74 communicates on its output side with the steering-control valve unit 10 through a port 77. First and second directional balanced valves 78 and 79 are axially slidably received in the bores 73 and 74, respectively. The first balanced valve 78 has a land 80 and a constriction 81 and, likewise, the second balanced valve 79 has a land 82 and a constriction 83, as illustrated. The lands 80 and 82 respectively have inner smaller working faces 80a and 82a and outer large working faces 80b and 82b. The constrictions 81 and 83 provide adjustable flow restrictions or orifices 84 and 85, respectively, which are defined by circumferential edges of associated stepped portions of the bores 73 and 74 and cooperating circumferential edges at which the constrictions terminate. An intermediate passage 86 provides fluid communication between the bores 73 and 74 around these constrictions 81 and 83, respectively, so that an output side of the first booster section 66 is hydraulically connected to an input side of the second booster section 67.

First and second fluid pressure acting chambers 87 and 88 are formed in contact with the outer faces 80b and 82b of the lands 80 and 82, respectively. The first pressure acting chamber 87 communicates with the intermediate passage 86 through a passage 89 while the second pressure acting chamber 88 communicates with the port 77 through a passage 90. The balanced valves 78 and 79 are thus subjected to forces resulting from 

1. A hydraulic-power control device for controlling hydraulic power to be supplied by a constant delivery pump to a hydraulic reaction chamber of a servo mechanism of a power-assisted steering system of a motor vehicle, which device comprises a steering-control valve unit which is responsive to steering resistance from front wheels of the motor vehicle and which is hydraulically connected to said pump for controlling the fluid pressure of the pump delivery to develop a steering-assistance fluid pressure in accordance with the steering resistance, speedsensitive control means responsive to driving speed of the motor vehicle and producing an electric signal which is representative of the vehicle driving speed, a pressure control valve unit responsive to said steering-assistance fluid pressure and to said electric signal and producing a control signal which is related to said steering-assistance fluid pressure and said electric signal, and a booster unit hydraulically connected to said pump, reaction chamber and steering control valve unit for varying a fluid pressure to be directed to said reaction chamber in accordance with said control signal.
 2. A hydraulic-power control deviCe according to claim 1, in which said pressure control valve unit is hydromechanically connected to said booster unit and said steering-control valve unit for supplying to the booster unit a mechanical signal as said control signal in accordance with said steering-assistance fluid pressure and said electric signal.
 3. A hydraulic-power control device according to claim 2, in which said booster unit comprises first and second balanced valves each having opposed, smaller and larger working faces, the smaller working faces being subjected to respective input pressures to the valves and the larger working faces being subjected to respective output pressures from the valves, the input pressure to the first balanced valve and the output pressure from the second balanced valves corresponding respectively to said pump delivery and steering-assistance fluid pressures, a first passageway for providing fluid communication between an input side of said first balanced valve and an output side of said second balanced valve through output and input sides of said first and second balanced valve respectively, said first passageway being provided with first and second adjustable flow restrictions intervening between the input and output sides of said first and second balanced valves respectively, said flow restrictions being adjustable by movements respectively of said first and second balanced valves due to differences between the input and output pressures acting upon the respective valves, and a second passageway providing interruptible fluid communication between the output sides of said first and second balanced valves for varying each of the output pressure from said first and second balanced valves in accordance with said mechanical signal which is supplied by said pressure control valve unit.
 4. A hydraulic-power control device according to claim 3, in which said pressure control valve unit comprises first and second electromagnetic actuating means which are energized in accordance with said electric signal supplied from said speed-sensitive control means, and first and second fluid flow control valves operatively connected to said first and second electromagnetic actuating means, respectively, each for being actuated when the associated actuating means is energized, said first and second fluid flow control valves being positioned in association with said output sides of said first and second balanced valves of the booster unit for controlling said second passageway to vary each of the output pressures from said first and second balanced valves.
 5. A hydraulic-power control device according to claim 1, in which said pressure control valve unit is hydraulically connected to said booster unit and said steering-control valve unit and supplying to said booster unit a signal fluid pressure as said control signal in accordance with said steering-assistance fluid pressure and said electric signal.
 6. A hydraulic-power control device according to claim 5, in which said pressure control valve unit comprises a first valve member which is operable to cut off said steering-assistance fluid pressure to be directed to said booster unit when actuated, a second valve member which is operable to relieve said steering-assistance fluid pressure at a limited rate when actuated, and first and second electromagnetic actuating means connected to said speed-sensitive control means and respectively associated with the first and second valve members for actuating these valve members when energized, said first electromagnetic actuating means being energized when said electric signal represents a vehicle driving speed which is lower than a predetermined relatively low level and said second electromagnetic actuating means being actuated when said electric signal represents a vehicle driving speed higher than a relatively high level.
 7. A hydaulic-power control device according to claim 6, in which said pressure control valve unit further comprises a first passageway which is hydrauliCally connected at its inlet side to said booster unit and at its outlet side to said steering-control valve unit through a flow restriction, said first valve member being positioned to shut-off said first passageway posterior to said flow restriction when actuated, and a second passageway leading at one end from said first passageway through a flow restriction and at the other opened to the outside of the pressure control valve unit, said second valve member being positioned to shut off said second passageway posterior to the flow restriction therein.
 8. A hydraulic-power control device according to claim 6, in which speed-sensitive control means comprises a vehicle speed detector for detecting said driving speed of the motor vehicle and producing a signal voltage substantially proportional to the detected driving speed and an elecric control circuit which is connected at its input terminal to said vehicle speed detector and at its output terminal to said first and second electromagnetic actuating means for energizing the first electromagnetic means when said signal voltage represents said vehicle driving speed lower than said relatively low level and energizing the second electromagnetic means when said signal voltage represents said vehicle driving speed higher than said relatively high level.
 9. A hydraulic-power control device according to claim 8, in which said electric control circuit comprises first and second Schmidt trigger circuits respectively having trigger levels corresponding to said relatively low and relatively high levels, said second Schmidt trigger circuit being connected to said second electromagnetic actuating means, and a NOT-gate circuit connected at its input terminal to said first Schmidt trigger circuit and at its output terminal to said first electromagnetic actuating means.
 10. A hydraulic-power control device according to claim 5, in which said booster unit comprises a directional balanced valve having opposed, larger and smaller working faces, the larger working face being subjected to signal fluid pressure and the smaller working face being subjected to the pressure to said pump delivery, and a pressure-control passageway providing fluid communication between said pump and said steering-control valve unit through an adjustable flow restriction which is adjustable by movement of said directional balanced valve, said restriction being constricted or even closed when said balanced valve is moved by a force resulting from said signal fluid pressure acting upon said larger working face of the valve overcoming a force resulting from the pump delivery pressure acting upon said smaller working face of the valve for giving rise to an increase in the pump delivery pressure which is to be directed to said reaction chamber.
 11. A hydraulic-power control device according to claim 10, in which said booster unit further comprises a valved passageway leading at one end from a pump side of said pressure-control passageway and opened at the other to the outside for draining off an excess of the pump delivery when the pump delivery pressure rises beyond a predetermined level.
 12. A hydraulic-power control device according to claim 11, in which said booster unit further comprises a spring-loaded pressure relief valve interposed between ends of said valve passageway to bias the valved passageway to be closed and to open the valved passageway when a pressure difference thereacross exceeds a predetermined level.
 13. A hydraulic-power control device according to claim 12, in which said pressure relief valve has self-maintaining characteristics by which the pressure relief valve is held at rest while said pressure difference is greater than a predetermined level.
 14. A hydraulic-power control device according to claim 10, in which said booster unit further comprises a valved passageway leading at one end from a steering-control valve unit side of said pressure-control passageway and opened to the outside at its other end for Draining off an excess of the steering-assistance fluid when the pressure thereof rises beyond a predetermined level.
 15. A hydraulic-power control device according to claim 14, in which said booster unit further comprises a pressure spring-loaded pressure relief valve interposed between ends of said valve passageway to bias the valved passageway to be closed and to open the valved passageway when a pressure difference thereacross exceeds a predetermined level.
 16. A hydraulic-power control device according to claim 15, in which said pressure relief valve has self-maintaining characteristics by which the pressure relief valve is held at rest while said pressure difference is greater than a predetermined level.
 17. A hydraulic-power control device for controlling hydraulic power to be supplied from a constant delivery pump to a hydraulic reaction chamber of a servo mechanism of a power-assisted steering system of a motor vehicle, comprising a steering-control valve unit which is responsive to steering resistance from front wheels of the motor vehicle and which is hydraulically connected to said pump for controlling the fluid pressure of the pump delivery to develop a steering-assistance fluid pressure in accordance with the steering resistance, speed-sensitive control means which is responsive to a driving speed of said motor vehicle for producing an electric signal representative of the vehicle driving speed, a pressure control valve unit hydraulically connected to said steering-control valve unit and electrically connected to said speed-sensitive control means for producing a signal fluid pressure in accordance with the steering-assistance fluid pressure and said electric signal supplied therefrom, and a booster unit hydraulically connected to said pump, reaction chamber, steering-control valve unit and pressure control valve unit for multiplying a fluid pressure which is to be directed to said reaction chamber in accordance with said signal fluid pressure when said vehicle driving speed represented by said electric signal is higher than a predetermined level.
 18. A hydraulic-power control device according to claim 17, in which said booster unit comprises a housing structure having formed therein fluid lines including a port communicating with said pump and said reaction chamber, a port communicating with said steering-control valve unit and with an input side of said pressure control valve unit, a port communicating with an output side of said pressure control valve unit and a bore into which all the said ports are open, and a directional balanced valve axially slidably received in said bore and having a land which has a smaller working face sub-jected to the pump delivery pressure through said port communicating with said pump and a larger working face subjected to said signal fluid pressure through said port communicating with said output side of said pressure control valve unit, said balanced valve defining in said bore an adjustable orifice which is closed and opened as said balanced valve is moved due to difference between opposing forces resulting from the fluid pressure acting upon the smaller and larger working faces of said land, said orifice providing communication between the ports communicating respectively with said pump and said steering-control valve unit when opened.
 19. A hydraulic-power control device according to claim 18, in which said booster unit further comprises first and second bypass passageway each opened at one end to the outside of said housing structure, said first bypass passageway bypassing said orifice and said second bypass passageway leading from said port commu-nicating with said steering-control valve unit, and first and second spring-loaded pressure relief valves disposed in said first and second bypass passageways respectively, each of said pressure relief valves being biased to close the associated bypass passage and moved to open the bypass passageway when the fluid pressure obtaining anterior thereto rises beyOnd a predetermined level for draining off an excess of the fluid pressure.
 20. A hydraulic-power control device according to claim 17, in which said pressure control valve unit comprises a housing structure having formed therein a first fluid passage having an inlet port communicating with an output side of said booster unit and an outlet port communicating with an input side of said booster unit, a flow restriction intervening between the inlet and outlet ports of said first fluid passage and a second fluid passage communicating through a flow restriction with said first fluid passage posterior to the flow restriction therein and opened to the outside of said housing structure, first and second flow shut-off valves projecting respectively into first and second fluid passages posterior to the respective flow restrictions and biased to open said first and second fluid passages, and actuating means associated with said flow shut-off valves and electrically connected to said speed-sensitive control means for moving each of said shut-off valves in accordance with said electric signal.
 21. A hydraulic-power control device according to claim 20, in which said actuating means comprises first and second solenoid devices respectively having plungers engaging with said first and second flow shut-off valves and having retracted positions when the respective solenoid devices are de-energized, said plungers being protruded when the respective solenoid devices are energized by said electric signal for moving the associated flow shut-off valves to close said first and second fluid passages independently of each other.
 22. A hydraulic-power control device according to claim 21, in which said speed-sensitive control means comprises a vehicle-pseed detector responsive to the driving speed of the motor vehicle and producing a d.c. signal voltage proportional to the detected vehicle driving speed, converting means for converting said d.c. signal voltage into an a.c. signal voltage, and switch means which is connected to said converting means and which is operable to produce said electric signal to energize said first solenoid device only when said signal voltage represents a vehicle driving speed lower than a predetermined relatively low level and to energize said second solenoid device only when said signal voltage represents a vehicle driving speed which is higher than a predetermined relatively high level.
 23. A hydraulic-power control device according to claim 22, in which said switch means comprises first and second Schmidt trigger circuits having trigger levels respectively corresponding to said predetermined relatively low and relatively high levels, said second Schmidt trigger circuit being connected to said second solenoid device, and a NOT-gate circuit connected between said first Schmidt trigger circuit and said first solenoid device.
 24. A hydraulic-power control device for controlling hydraulic power to be supplied from a constant delivery pump to a hydraulic reaction chamber of a servo mechanism of a power-assisted steering system of a motor vehicle, comprising a steering-control valve unit which is responsive to steering resistance from front wheels of the motor vehicle and which is hydraulically connected to said pump for controlling the pump delivery pressure to develop steering-assistance fluid pressure in accordance with said steering resistance, speed-sensitive control means which is responsive to a driving speed of said motor vehicle for producing an electric signal representative of the vehicle driving speed, a pressure control valve unit hydromechanically connected to said steering-control valve unit and electrically connected to said speed-sensitive control means for producing a mechanical signal in accordance with said electric signal, and a booster unit hydraulically connected to said pump, reaction chamber and steering-control valve unit and hydromechanically connected to said pressure control valve unit for multiplying a fluid pressure Which is to be directed to said reaction chamber when said vehicle driving speed represented by said electric signal is higher than a predetermined level.
 25. A hydraulic-power control device according to claim 24, in which said booster unit comprises a housing structure having formed therein a port communciating with said pump and reaction chamber, a port communicating with said steering-control valve unit, a first bore anteriorly communicating with said port communicating with said pump and reaction chamber, a second bore posteriorly communicating with said port communicating with said steering-control valve unit, said first and second bores communicating with each other through an intermediate passage, a first pressure acting chamber anteriorly communicating with said port communicating with said pump and posteriorly communicating with said intermediate passage, a second pressure acting chamber communicating anteriorly with said first pressure acting chamber and said intermediate passage and posteriorly with said port communicating with said steering-control valve unit and first and second directional balanced valves respectively axially slidably received in said first and second bores, each of said balanced valves having a land and a constriction defining an adjustable flow restriction in the associated bore, the land of said balanced valve having a smaller working face subjected to the pump delivery pressure and a larger working face subjected to a fluid pressure in said first pressure acting chamber and the land of said second balanced valve having a smaller working face subjected to a fluid pressure in said intermediate passage and a larger working face subjected to a fluid pressure in said second pressure working chamber the adjustable flow restriction in said first bore intervening between said port communicating with said pump and said intermediate passage and the adjustable flow restriction in said second bore intervening between said intermediate passage and said port communicating with said steering-control valve unit, each of the adjustable flow restrictions being closed and opened as the associated balanced valve is axially moved due to difference between opposing forces resulting from the fluid pressures acting on both working faces of its land, said pressure control valve comprising first and second spring-loaded fluid shut-off valves projecting toward said first and second pressure acting chambers, respectively, said first fluid shut-off valve being axially movable between postions establishing and interrupting fluid communication between said first pressure acting chamber and said port communicating with said pump and biased to establish such communication and said second fluid shut-off valve being axially movable between positions establishing and interrupting fluid communication between said second pressure acting chamber and said port communicating with said steering-control valve and biased to interrupt such commnication, and first and second valve actuating means associated with said first and second fluid shut-off valves and electrically connected to said speed-sensitive control means for moving said first fluid shut-off valve to interrupt said fluid communication and moving said second fluid shut-off valve to establish said fluid communication when energized by said electric signal.
 26. A hydraulic-power control device according to claim 24, in which said booster unit comprises a housing structure having formed therein a port communicating with said pump and reaction chamber, a port communicating with said steering control valve unit, a first bore anteriorly communicating with said port communicating with the pump and reaction chamber, a second bore posteriorly communicating with said port communicating with said steering-control valve unit, said first and second bores communicating with each other through an intermediate passage, first and second pressure acting chambers communicating respectively with said intermediate pAssage and said port communicating with said steering-control valve unit, and a drain passage having one end opened to the outside of said housing structure and branch ends disposed adjacent said first and second pressure acting chambers, and first and second directional balanced valves respectively axially slidably received in said first and second bores, each of said balanced valves having a land and a constriction defining an adjustable flow restriction in the associated bore, the land of said first balanced valve having a smaller working face subjected to the pump delivery pressure and a larger working face subjected to the fluid pressure in said first pressure acting chamber and the land of said second balanced valve having a smaller working face subjected to the fluid pressure in said intermediate passage and a larger working face subjected to the steering-assistance fluid pressure in said port communicating with said steering-control valve unit, the adjustable flow restriction in said first bore intervening between said port communicating with said pump and said intermediate passage and the adjustable flow restriction in said second bore intervening between said intermediate passage and said port communicating with said steering-control valve unit, each of the adjustable flow restrictions being closed and opened as the associated balanced valve is axially moved due to difference between opposing faces resulting from the fluid pressures acting upon both working faces of its land, said pressure control valve unit comprising first and second spring-loaded fluid shut-off valves projecting toward said first and second pressure acting chambers and axially movable between positions to close and open the chambers, respectively, each of said fluid shut-off valves being formed with a fluid passage having one end open to the associated pressure acting chamber and the other end opened and positioned to be in alignment with each of said branch ends of said drain passage when the fluid shut-off valve is in a position to close the associated pressure acting chamber, said first fluid shut-off valve being biased to open said first pressure acting chamber and said second fluid shut-off valve being biased to close said second pressure acting chamber, and first and second valve actuating means associated with said first and second fluid shut-off valves and electrically connected to said speed-sensitive control means for moving said first fluid shut-off valve to a position to close said first pressure acting chamber and moving said second fluid shut-off valve to open said second pressure acting chamber when energized by said electric signal.
 27. A hydraulic-power control device according to claim 26, in which said booster unit further comprises first and second bypass passageways which are opened each at one end to the outside of said housing structure, said first bypass passageway bypassing said first adjustable flow restriction and said second bypass passageway leading from said port communicating with said steering-control valve unit, and first and second spring-loaded pressure relief valves disposed respectively in said first and second bypass passageways, each of said pressure relief valves being biased to close the associated bypass passageway and moved to open the passageway when the fluid pressure obtaining anterior thereto rises beyond a predetermined level for draining off an excess of the fluid pressure therefrom.
 28. A hydraulic-power control device according to claim 26, in which said speed sensitive control means comprising a vehicle-speed detector responsive to the driving speed of the motor vehicle and producing a d.c. signal voltage proportional to the detected vehicle driving speed, converting means for converting said d.c. signal voltage into an a.c. signal voltage, and switch means which is connected to said converting means and which is operable to produce said electric signal to selectively energize said first and second valve actuating means in aCcordance with said a.c. signal voltage.
 29. A hydraulic-power control device according to claim 28, in which said switch means comprises first, second, third and fourth Schmidt trigger circuits connected to said converting means, said first and third Schmidt trigger circuits having a common trigger level which is in agreement with a predetermined reference voltage, said second Schmidt trigger circuit having a trigger level lower than said common trigger level and said fourth Schmidt trigger circuit having a trigger level which is higher than said common trigger level, said first Schmidt trigger circuit being connected to said second valve actuating means, first and second NOT-gate circuits connected to said second and fourth Schmidt trigger circuits, an AND-gate circuit connected to said third Schmidt trigger circuit and said second NOT-gate circuit, and an OR-gate circuit connected at its input terminal to said first NOT-gate circuit and said AND-gate circuit and at its output terminal to said first valve actuating means. 